Hi, Joanne Rowling has told in her Twitter account that the gems
for the Hufflepuff Housepoints are:

Diamonds

This is what she wrote:

As well as that reminder, Rowling also answered a fan’s question
regarding the gems in the Hufflepuff hour glass. @HarmonicalHero
Diamonds, of course. Because that’s what Hufflepuffs are (some a
little rougher than others). X

Hi, now where the Warner Bros. Studio Tour - The Making of
Harry Potter is opened, it would interest me if someone will go
to it. I myself live in Germany, so it's not so easy for me. But
perhaps some Londoners can go and tell us from that. I would
appreciate.